Power over Ethernet (PoE) (according to the IEEE 802.3-2005 standard) provides a means for network devices to be powered over the network cabling. In a typical PoE configuration, several PoE Powered Devices (PDs) are powered by Power Sourcing Equipment (PSE). Each PD is often allocated the maximum power allowed by the specification (approximately 12.95 Watts, drawing up to 350 mA of current over a twisted pair connection at between 44 and 57 Volts—note that the cable introduces some power dissipation due to resistance, so the amount of power supplied by the PSE is about 15.4 W at 44 V). Thus, for example, a PSE having a 154 W power supply is able to power 10 PDs allocated 12.95 W each.
It sometimes happens that a voltage spike (or any sort of rapid voltage change) can occur, where the voltage of the PSE increases by a small amount very rapidly. Because PDs typically contain capacitors, the rapid voltage change causes the capacitors to charge and therefore draw excess current. Because a power supply is typically able to provide 115% of its stated power capacity for brief periods of time (e.g., 50 ms), a brief 15% increase in current is acceptable. Since PoE per-port current is usually limited to 400 mA, however, a voltage spike will not damage the power supply because 400 mA of current is less than 15% more than the 350 mA allocated to each port.
The newer IEEE 802.3at (also known as PoE+) draft standard increases the power available over the traditional PoE. Thus, according to one proposal, each port can provide 720 mA of continuous current, permitting excursions of up to approximately 15% more current. However, most PoE+ implementations thus far have limited the per-port current to less than 820 mA. Thus, if all devices are allocated 720 mA, a voltage spike will typically not cause a failure because 820 mA is approximately 15% more than 720 mA. Some pre-standard PoE+ implementations are configurable to have varying current limits because the PoE+ standard has not yet been completed. Thus, for example, some such implementations allow the current limit to be changed to comply with the specification once finalized (by a flash hardware update or reprogramming).